WASHINGTON — When President Obama nominates Chuck Hagel, the maverick Republican and former senator from Nebraska, to be his next secretary of defense, he will be turning to a trusted ally whose willingness to defy party loyalty and conventional wisdom won his admiration both in the Senate and on a 2008 tour of war zones in Iraq and Afghanistan.

The choice of Mr. Hagel, the first Vietnam veteran to be nominated for the post, would add a prominent Republican to Mr. Obama’s cabinet, providing some political cover for the president’s plans to exit Afghanistan and make cuts to a military budget that has roughly doubled since the 2001 terrorist attacks.

But Republicans made clear on Sunday that they would give Mr. Hagel a rough ride on his path to the Pentagon, questioning his support for Israel, his seriousness about the Iranian nuclear threat and his commitment to an adequate defense budget. And Mr. Obama may also face difficulties from some Democrats who are wary of negative comments that Mr. Hagel made more than a decade ago about gays.

Some Obama aides had doubts about the wisdom of the choice, given Mr. Hagel’s frosty relationship with members of his own party, but officials said they were confident that they could corral enough votes from both sides of the aisle to win confirmation in the Senate. White House officials confirmed on Sunday that Mr. Hagel was Mr. Obama’s pick for the job and said the announcement would come as early as Monday.

Rather than turning to a defense technocrat, Mr. Obama decided on an independent politician whose service in Vietnam gave him a lifelong skepticism about the commitment of American lives in overseas conflicts. Like Mr. Obama, Mr. Hagel supported the war in Afghanistan but opposed the troop surge in Iraq under President George W. Bush.

Mr. Hagel, 66, served as an enlisted man in Vietnam, won two Purple Hearts and still carries bits of shrapnel in his chest. He was the co-founder of a cellular telephone company and headed an investment banking firm before being elected to the Senate in 1996. He retired in 2009 and now teaches at Georgetown University and serves as chairman of the Atlantic Council, a centrist foreign policy group.

In July 2008, during the presidential campaign, Mr. Hagel accompanied Mr. Obama, who was then in the Senate, on a six-day trip to Afghanistan, Iraq, Jordan and Kuwait. When Senator John McCain, the Republican presidential nominee that year, asserted that Mr. Obama’s motive for the trip was political, Mr. Hagel strongly defended Mr. Obama, saying in a television interview that Mr. McCain was “on thin ground” in trying to impugn Mr. Obama’s patriotism.

In the Senate, Mr. Hagel voted in favor of the resolution authorizing Mr. Bush to take military action in Iraq, which passed overwhelmingly in October 2002. But he soured on the effort early, and became an advocate of the view that America had lost sight of what it was trying to accomplish and that it was overestimating its ability to change Iraqi society.

In some ways, Mr. Hagel bears a similarity to Susan E. Rice, the ambassador to the United Nations and Mr. Obama’s first choice for secretary of state. She withdrew her name from consideration, making way for the selection of Senator John F. Kerry. Ms. Rice, like Mr. Hagel, is a trusted Obama ally who spoke up for him during the 2008 presidential campaign and became a lightning rod for Republican attacks.

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President Obama and his family arriving back at the White House on Sunday.Credit
Pool photo by Dennis Brack

“The president wants someone whose judgment he respects on the big questions of war and peace,” said Philip D. Zelikow, a senior State Department official under Mr. Bush and now a member of the President’s Intelligence Advisory Board. Mr. Hagel is co-chairman of the board.

The White House is calculating that opposition to Mr. Hagel may be loud but not broad and that in end the Senate will confirm him. Administration officials argued that voting against a Republican war hero to run the Defense Department would not be easy for fellow Republicans, and they are confident that disgruntled Democrats will ultimately not deny their president his choice.

“At the end of the day, Republicans will support a decorated war hero who was their colleague for 12 years and has critical experience on veterans’ issues,” said an administration official who requested anonymity to discuss a nomination before it was announced. “It would be hard to explain a no vote just because he bucked his party on Iraq, a war most Americans think was a disaster.”

When he took office in 2009, Mr. Obama asked Robert M. Gates, the defense secretary during Mr. Bush’s last two years in office, to remain in his job. Mr. Gates, a former C.I.A. chief and deputy national security adviser, belonged to the mainstream of Republican defense orthodoxy. Mr. Hagel does not, as was evident in harsh comments from Republicans on Sunday.

Senator Lindsey Graham, Republican of South Carolina, said he personally liked Mr. Hagel but considered him “out of the mainstream of thinking on most issues regarding foreign policy.”

“This is an in-your-face nomination of the president to all of us who are supportive of Israel,” Mr. Graham said on the CNN program “State of the Union.” “I don’t know what his management experience is regarding the Pentagon — little, if any — so I think it’s an incredibly controversial choice.”

The Senate minority leader, Mitch McConnell, said on the NBC News program “Meet the Press,” “I think there will be a lot of tough questions for Senator Hagel, but he will be treated fairly by Republicans in the Senate.”

For weeks, some Jewish groups sought to dissuade Mr. Obama from choosing Mr. Hagel, who once referred to advocates of Israel as “the Jewish lobby.” Having failed, opponents over the weekend shifted to trying to block Mr. Hagel’s confirmation.

Regional chapters of the American Jewish Committee, which has bipartisan bona fides, began circulating letters to their Democratic senators, urging them to oppose Mr. Hagel.

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Chuck Hagel is a Republican but a maverick, and his nomination is likely to be opposed by some members of his own party.Credit
Chip Somodevilla/Getty Images

One such letter, obtained by The New York Times, said: “While AJC recognizes Senator Hagel’s record of service to our country and the people of Nebraska, his opinions on a range of core U.S. national security priorities run counter to what AJC advocates and what President Obama has articulated — notably, on the efficacy of Iran sanctions, on a credible military option against Iran, on branding Hezbollah as terrorist organization, and on the special nature of the U.S.-Israel relationship.”

Mr. Hagel and his supporters have dismissed criticism of his views on Israel, noting that he voted on several occasions to provide billions of dollars in military aid to the country. He also was a sponsor of legislation that urged the international community to avoid contact with Hamas until it recognized Israel’s sovereignty.

On one of the biggest security challenges the administration faces — how to slow or stop Iran’s progress toward a nuclear capability — Mr. Hagel’s views appear somewhat at odds with the president’s. White House aides have been seeking to minimize the differences in advance of the expected nomination.

Mr. Hagel has long been an opponent of unilateral American sanctions against Iran — among other American adversaries — viewing them as counterproductive. Notably, he was one of only two senators to vote against the Iran-Libya sanctions act in 2001, arguing that it would undercut efforts to engage with Tehran.

But today, the administration describes the policy of tough sanctions against Tehran as the key to its strategy for forcing the country’s leadership to reverse course on its nuclear program.

As secretary of defense, Mr. Hagel would not be directly involved in designing or enforcing those sanctions; that is the work of the Treasury and State Departments. But he would be in charge of one of the other major elements of pressure: the huge buildup of American naval might, antimissile capability and special operations in the Persian Gulf. That force is intended not only to keep the Strait of Hormuz open but also to make credible Mr. Obama’s threat to use military force to prevent Iran from obtaining nuclear weapons.

“So far, Obama’s big problem is that the threat to use force has not seemed credible,” a former official who has worked on Iran issues with Mr. Hagel and frequently advises the administration on Iran said last week. “The question is whether if Chuck is defense secretary, the Iranians would take seriously the thought that he is willing to use force if it comes to that.”

In efforts to spur liberals to oppose the nomination, Mr. Hagel’s critics have also focused on a comment he made in the late 1990s, opposing a Clinton administration ambassadorial nominee for being “openly, aggressively gay,” and his past stances on gay rights issues.

Mr. Hagel has since apologized for the remark and in a recent statement said he supported the right of gays and lesbians to serve openly in the military. Gay rights groups like the Human Rights Campaign that tend to lean Democratic have not yet taken a position on Mr. Hagel’s nomination. But another gay group, the Log Cabin Republicans, has been vocal in its opposition to Mr. Hagel.

White House officials noted that Mr. Hagel apologized for comments offending Israel backers and said opposing him because of his Iraq stance would not help war hawks since Mr. Obama, who opposed the invasion from the start, would simply pick another like-minded nominee. As for his opposition to a gay nominee, officials said it would be hard to imagine a Nebraska Republican whose views had not evolved in the last decade.

Correction: January 8, 2013

Because of an editing error, an article on Monday about the selection of Chuck Hagel, a former Republican senator from Nebraska, to become defense secretary misidentified the Senate post held by Senator Mitch McConnell, a Republican of Kentucky, who said that Mr. Hagel would be questioned toughly but treated fairly during confirmation hearings. Mr. McConnell is the minority leader, not the majority leader. (That position is held by Harry Reid, Democrat of Nevada.)

Nicholas Confessore and Michael Schwirtz contributed reporting from New York, and Peter Baker from Washington.

A version of this article appears in print on January 7, 2013, on page A1 of the New York edition with the headline: Hagel Said To Be Defense Choice; Battle Foreseen. Order Reprints|Today's Paper|Subscribe